Recently, as use of digital video increases rapidly, starting from video search by means of video indexing, or content basis search by using an image, development of a variety of multimedia service systems has been made. Most of content basis search systems by using images use a method in which proper characteristic vectors are abstracted from the images, and a similarity of the images are determined with reference to a similarity of the characteristic vectors. Of different image characteristics, an HSV (Hue-Saturation-Value) Color Histogram is used widely as the most typical image characteristic.
There are a variety of methods for expressing a color, by using different color models depending on application, such as RGB (Red, Green, and Blue; a color model for expressing a color with three primary color of red, green, and blue), CMY (Cyan, Magenta, and Yellow; a typical color model in a printing field), YCbCr (Gamma transformed luminance/color difference separation color model), HSV (typical color model used in a Computer Graphic field), and the like.
Of the different color models, the YCbCr color model, very efficient in compressing a picture data, is used in most of picture compressing international standards, such as JPEG (Joint Photographic Coding Experts Group), MPEG (Moving Picture Experts Group), and the like. The YCbCr color model indicates ratios of three components of luminance information ‘Y’, color difference information ‘Cb’, and ‘Cr’ included in a horizontal line of a TV screen and the like, wherein the luminance of a pixel is represented with 8 bits, and the color of the pixel is represented with the color difference information by using two 8 bits.
However, since the YCbCr color model is far from human perception characteristics, the HSV color model, which reflects the human perception characteristics the best, is mostly used in abstracting the characteristic vectors from the picture. That is, a YCbCr color space is converted into an HSV color space of a Hue, Saturation, and Value before using the YCbCr color.
FIG. 1 illustrates a block diagram of a related art HSV color histogram abstractor.
The related art HSV color histogram abstractor, for abstracting a HSV color histogram from the YCbCr color model, is provided with a YCbCr/HSV converter 101 for converting a pixel value from the YCbCr color model to the HSV color model, a quantizer 102 for quantizing the HSV color model converted at the YCbCr/HSV converter 101 to provide a histogram index, and a histogram generator 103 for increasing a histogram BIN value of the histogram index from the quantizer 102 by one, to generate a histogram.
A related art method for abstracting an HSV color histogram from the YCbCr color model will be described.
For abstracting the HSV color histogram from the picture stored in the YCbCr color model, the YCbCr/HSV converter 101 converts a pixel value from the YCbCr color model to the HSV color model with the following equations.
                              H          =                                                    tan                                  -                  1                                            ⁢                                                Cr                  ⁢                                      -                                    ⁢                  128                                                  Cr                  ⁢                                      -                                    ⁢                  128                                            ⁢              S              ⁢                              180                π                                      -            180                          ,                  0          ⁢                                          ⁢          D          ⁢                                          ⁢          HD          ⁢                                          ⁢          360                                    (        1        )                                          S          =                                                                      (                                      Cr                    ⁢                                          -                                        ⁢                    128                                    )                                2                            +                                                (                                      Cb                    ⁢                                          -                                        ⁢                    128                                    )                                2                                                    ,                  0          ⁢          D          ⁢                                          ⁢          SD          ⁢                                          ⁢          128                                    (        2        )                                          V          =          Y                ,                  0          ≤          V          ≤          255                                    (        3        )            
Then, the quantizer 102 quantizes the pixel value of the HSV model, to obtain a histogram index. As an example of an actual quantization which can be used effectively in the content basis image search application, a case of quantization as shown in FIG. 3 will be described.
Referring to FIG. 3A, the HSV model is represent with a three dimensional (HSV) cylindrical coordinate system, in which an axis of the cylinder represents ‘V’, a concentric circular direction starting from a center (0) represents ‘S’, and an angle represents ‘H’. A method for quantizing the three dimensional HSV space represented thus is illustrated in sections of an HS plane and an SV plane.
That is, a pixel, having an ‘S’ equal to or smaller than 5, is regarded as a gray scale, and quantized only with respect to ‘V’ in four stages each with 64 levels regardless of an H value.
A color, having the ‘S’ greater than 5 and equal to or smaller than 30, is quantized with respect to ‘H’ in six stages each with 60 degrees, and ‘V’ in two stage each with 128 levels. A color, having the ‘S’ greater than 30, is quantized only with respect to ‘H’ in six stages each with 60 degrees while disregarding ‘V’.
A part having the S greater than 30 is quantized coarser than a part having the S smaller than 30 for reflecting a probability that occurrence of great S is relatively small in natural pictures. Thus, a histogram having 22 BINs (four stages with respect to ‘V’ for a pixel having S smaller than 5, six stages with respect to ‘H’ and two stage with respect to ‘V’ (2×6) for a color having an S greater than 5 and smaller than 30, and six stages with respect to ‘H’ for a color having an S greater than 30) is drawn up. The quantizer 102 determines a given pixel value of being a point belonging to which of 22 sections divided by circular, and radial boundaries in the three dimensional HSV space, and provides an index value between 0 to 21 representing the section. Every time the quantization index is obtained, the histogram generating part 103 increases a BIN value of the histogram for the index by one, to draw up the histogram.
However, the related art method for abstracting an HSV color histogram from a picture in the YCbCr color model has the following problems.
That is, even though the HSV color histogram is drawn up if the foregoing process is carried out for all pixels, the square root operation and the arctan function operation in the equations (1) and (2) for converting the YCbCr color model to the HSV color model costs very high.